These capuchins are abducting babies from howler monkeys—for fun?

On a tiny island off the coast of Panama called Jicarón, a male capuchin monkey called Joker appears to have started a disturbing trend.

Camera traps caught Joker, nicknamed for the scar on his face, and other male white-faced capuchins (Cebus capucinus imitator) carrying kidnapped howler monkey infants on their backs. Researchers originally set up the traps in 2017 after a botanist visiting the island had reported the monkeys using stones to process food, which had never been seen before in the more slender kind of capuchin that inhabits Costa Rica and Panama. The cameras did reveal one group of capuchins using stone tools and anvils to crack open seeds, fruits, even crabs and snails.

Yet as the team reports in the journal Current Biology this week, the footage also captured this bizarre baby-snatching fad, something never seen before. “It was so weird that I went straight to my advisor’s office to ask him what it was,” says primatologist Zoë Goldsborough of the Max Planck Institute of Animal Behavior and the Smithsonian Tropical Research Institute. 

(Capuchins are known for their ingenuity—another species is 3,000 years into its own “Stone Age”.) 

A bizarre new trend

A recording dated January 26, 2022, first documented an unidentified young capuchin male carrying a howler monkey (Alouatta palliata coibensis) infant. The next day Joker was carrying that same infant. And so he did for days on end, at least until February 3. “Our first thought was that maybe this infant had been abandoned by the howlers, and then adopted,” Goldsborough says.

There was one known case of a marmoset monkey infant being adopted by a different species of capuchin in Brazil. But crucially, that baby was adopted by a female who could nurse it, says Patrícia Izar of the University of São Paulo, who reported that finding in 2006. Capuchin males, on the other hand, don’t have a clue what to do. And so the kidnapped howler infant very likely died of starvation. 

What’s more, the poor infant was making the kind of calls it usually makes when separated from its mom—and later on, some adult howlers called out as well, indicating the infant had not been abandoned, but abducted instead. “We don’t have footage of how the capuchins did this,” says study coauthor Brendan Barrett, a behavioral ecologist at Max Planck. “But we know they are not afraid to gang up on much larger howlers.” 

Things were about to get a whole lot weirder. In April and May, Joker was seen carrying another howler infant, and then another. Footage also showed him dragging a third one, possibly dead, with some other young males tagging along. Then, between September and March, the situation escalated: Four other males were seen carrying live howler infants on their backs or bellies, sometimes for more than a week. Over a span of 15 months, at least 11 infants had been abducted—and few if any are likely to have survived.  

What drives the kidnapping?

While there had been at least one earlier report of a capuchin from a larger species in Brazil stealing a howler monkey infant and carrying it off in its mouth, presumably to eat it, this study is the first to document white-faced capuchins abducting infants in this way—and researchers are especially fascinated that the behavior was subsequently picked up by other individuals, too.

“This observation is particularly intriguing because examples of the social spread of such behaviors with no apparent fitness benefits in animals other than humans are rare,” says Izar. Because there had been camera traps on the island for years before this behavior was ever observed, the researchers probably captured the first time it happened, or at least a very early occurrence.

It is not unusual for young capuchin males to be seen carrying infants of their own species, says Susan Perry of the University of California, Los Angeles, who has studied Costa Rican capuchins at another site for decades, but was not involved in the current study. “Capuchin males often try to steal capuchin infants, and they seem extremely pleased—as if they’ve won a prize—when they succeed. Until the infant gets hungry and starts crying for milk.”

At that point, the infants tend to be abandoned. “Fortunately, the infant’s mother or other female relatives are usually lurking nearby to retrieve their infants.” Capuchin males have a preference for male infants, says Perry. “We think the infants they develop a close relationship with early on will often grow up to become allies with whom they can make the risky move to another group to mate.”

Abducting howler monkeys would obviously be useless in this regard, but perhaps their urge to carry infants is so strong that it sometimes misfires, says Perry. Goldsborough and Barrett agree, but they believe another tendency may be misfiring as well—the desire to do as others do. Perhaps Joker really just wanted to carry an infant, and then the others just wanted to have a go at it as well.

Not that it improved their social standing—young males carrying howler infants appeared to be the target of aggression from other capuchins more often than those that weren’t. But for a species in which learning a new technique to get your hands on difficult-to-reach but nutritious foods is an important part of growing up, perhaps the tendency to do as others do pays off often enough to be indiscriminate.

Island life may bore capuchins

The island environment could be a factor, too, the researchers argue. On the mainland, capuchins usually have to be wary of predators, and foraging takes up more time when you have to be constantly on guard and stay close to the group. On an island with plenty of food and hardly any threats, perhaps young males are just bored. “Animals living on islands with no predators—or in zoos, were they are also safe and well-fed—have often been found to be more innovative and better at using tools,” says Goldsborough.

In many cases, what bored animals come up with may be useless or even annoying, says Barrett. “I’ve seen capuchins groom porcupines and smack cows on the butt. They mess with everything. They’re just constantly testing and interacting with the world.” But occasionally, an individual will find that, hey, if you swing a rock at one of these smelly, colorful things on the beach, there’s a tasty treat inside. Or that if you hang out with male infants, they’ll have your back when they grow up.

Some capuchins also develop strange rituals with no other purpose than strengthening social bonds. It’s all in a day’s work for this large-brained, hypersocial, tirelessly inventive species that in many ways resembles our own, even though our last common ancestor lived around 38 million years ago. 

But what about the poor howlers, an endangered species on Jicarón, whose babies are being abducted? “It’s tragic,” says Goldsborough, “but as researchers, we don’t intend to interfere with natural behavior. I hope the howlers will eventually adapt, for example by keeping a safe distance from this one population of capuchins, or that the capuchins themselves will eventually tire of this. Those howler infants can be quite a handful.”